As shown in FIG. 1, a conventional projection system includes a solid-state light source 101, a rotary color wheel 102, a light-uniforming device 103, a spatial light modulator 104, and a projection lens 105. In particular, the solid-state light source 101 often adopts a blue light source, the rotary color wheel 102 at least includes a first region having a red phosphor R, a second region having a green phosphor G, and a third region having a blue phosphor B. When the rotary color wheel 102 is rotated around a shaft 1020, the blue light emitted by the solid-state light source 101 is incident onto different regions having different phosphors, exciting primary light of red color, primary light of green color, and primary light of blue color in a sequence of time. The primary light of red, green and blue color is mixed by the light-uniforming device 103 and modulated by the spatial light modulator 104, then project onto a screen 106 through the projection lens 105.
Currently, conventional spatial light modulators, such as Digital Micro mirror devices (DMD), often modulate the gray scale of images in a binary mode. For example, given the number of the gray scales is four, after the light source sequentially generates primary light of red color, green color, and blue color, the DMD divides the time duration of each primary light into four periods. For example, the time duration of the red primary light is divided into R1, R2, R3 and R4, the time duration of the green primary light is divided into G1, G2, G3 and G4, and the time duration of the blue primary light is divided into B1, B2, B3 and B4. The sequence of modulation periods is shown in FIG. 2. The four periods of the red primary light are modulated first, after which the four periods of the green primary light and the four periods of the blue primary light are modulated. In particular, the time duration of each period of the red primary light satisfies TR420=TR321=TR222=TR123, the time duration of each period of the green primary light satisfies TG420=TG321=TG222=TG123, and the time duration of each period of the blue primary light satisfies TB420=TB321=TB222=TB123.
In the above-mentioned projection system, the red, green, and blue primary light is generated by rotating the rotatory color wheel. That is, the switching speed of the primary light of different colors depends on the rotational speed of the rotatory color wheel, which results a substantially low switching speed of the primary light of different colors. Accordingly, the projected images may be subjected to a rainbow effect, degrading the quality of the projected images.